Hydrothermal processes in the Fen alkaline-carbonatite complex, southern Norway

We present detailed textural and chemical analyses of the hydrothermal sulfide- and REE-Th-Nb-mineralization observed in the Fen complex (southern Norway), which is the biggest carbonatite-related REE and Th deposit in Europe. The alkaline silicate rocks and carbonatites of the Fen complex underwent two hydrothermal alteration events that caused (i) the formation of sulfides and (ii) (re)-mobilization of REE, Fe and Al. This renders the Fen complex an ideal locality to study the genesis of hydrothermal sulfide and REE mineralization in carbonatites. Our observations record a hydrothermal alteration history of the Fen carbonatite complex, which is relevant to many carbonatite complexes worldwide. The first alteration event (pyrite-stage) caused the crystallization of sulfides (mostly pyrite) in all lithologies and was induced by a sulfide-rich fluid 1 that was probably derived from adjacent mafic alkaline silicate rocks. Veins formed during this hydrothermal event show a typical succession from magnetite via pyrite I + hematite + magnetite to pyrite II. A subsequent alteration event is characterized by the interaction of two evolving fluids, namely (1) a REE-rich fluid 2 that was probably derived from the carbonatites (autometasomatic fluid) and (2) an oxidizing meteoric fluid 3 that mainly introduced Si and was in equilibrium with the basement host rocks. The interaction of these two fluids with the carbonatitic rocks resulted in various characteristic types of calcite-hematite rocks locally called rodberg. During their late-stage silicification by fluid 3, P was mobilized from magmatic apatite and reprecipitated in close-by rodberg veins as small fluorapatite-monazite veins. The interaction of the carbonatite-derived fluid 2 and meteoric fluid 3 also resulted in the formation of distinct zones of LREE-phases, notably REE-F-carbonates (bastnasite, parisite, synchysite), monazite and allanite, and HREE-phases (including samarskite, aeschynite and an unnamed Nb-Fe-REE-Th-oxide. The most pronounced HREE-enrichment occurs in domains of strongest hydrothermal alteration, which was probably caused by preferential precipitation of HREE-rich complexes coupled with later partial leaching of LREE by F-rich fluids during silicification. Thorium-rich minerals were precipitated in close association with HREE-rich minerals implying similar hydrothermal behaviour of Th and HREE.